The present invention relates to a composition presenting a high resistance to the propagation of fire, and intended particularly but not exclusively to manufacturing electrical and/or optical cables for conveying power and/or transmitting information, which cables are required to withstand the high temperatures of a fire for a certain length of time.
The materials used as insulating and/or protecting coverings for cables that are resistant to the propagation of fire are conventionally made fireproof by incorporating a fire-retarding system in the polymer or the copolymer on which the covering is based.
Known fire-retarding systems are based in particular on:
halogen derivatives;
metal hydroxides, in particular magnesium hydroxide Mg(OH)2 and alumina trihydrate Al(OH)3; and
silicone, magnesium stearate, and calcium carbonate CaCO3, as described in document EP-0 774 487.
At present, the use of halogen derivatives is becoming less common because of the toxic and corrosive fumes they give off during combustion.
With systems using a combination of silicone, magnesium stearate, and calcium carbonate, the metal salt (magnesium stearate) migrates towards the surface of the base material in the event of a fire and decomposes, giving rise to an inorganic residue that reinforces the inorganic barrier of calcium silicate formed on the surface of the base material by the silicone and the filler of calcium carbonate. Nevertheless, those materials do not form ash that is sufficiently compact in the event of a fire. As a result the residue is friable, and therefore less effective. Fire protection remains limited and therefore does not comply with all standards.
Finally, although systems using metal hydroxides present acceptable fire resistance, they are nevertheless not suitable in all applications. Metal hydroxides must constitute a large fraction (at least 60% by weight) of the composition in which they are incorporated in order to obtain good resistance to fire. As a result, the viscosity of compositions containing such hydroxides as fireproofing additives is increased, thereby reducing the rate at which they can be extruded and consequently reducing the speed at which cables making use of them can be produced. Forming is therefore lengthier, and manufacturing costs are higher.
In addition, the high concentration of metal hydroxide, and in particular of alumina trihydrate, in compositions for use in making electrical cables degrades electrical insulation so that such compositions are usable only as the protective covering of electrical cables and not as the insulating covering of their conductors.
In addition, a high concentration of metal hydroxides degrades the mechanical properties of the compositions in which the hydroxides are incorporated.
The object of the present invention is thus to develop a fire-retarding composition suitable for being used as a protective sheath for cables without penalizing the manufacturing process and without penalizing the mechanical characteristics of such cables, and having an oxygen limit index (OLI) of 35%, typically at least as good as that of fire-resistant compositions based on metal hydroxides, and having fire-withstanding properties that are better than those of prior art compositions.
To this end, the present invention provides a composition possessing high resistance to fire propagation, the composition comprising a base polymer and a fireproofing additive containing a metal hydroxide,
wherein said fireproofing additive further contains a compound based on a carbonate of a metal from group IA of the periodic table of elements coated in a metal salt, said compound constituting at least 10% by weight of fireproofing additive.
The invention relies on using a material complying with a known type of fireproofing mechanism, i.e. a mechanism based on using metal hydroxides, and on incorporating in said material additives that are not intrinsically fireproofing but that act in synergy with the metal hydroxide and lead to an improvement in the mechanical characteristics of the final material and also to an improvement in the method of manufacturing cables having a sheath made of such a material. In addition, the fireproofing properties of materials obtained in this way are also improved.
In the invention, at least 10% of fireproofing additive is constituted by the compound based on a metal carbonate, such that the quantity of metal hydroxide in the composition is correspondingly reduced.
As a result, problems associated with the use of metal hydroxides are limited, and in particular degraded mechanical performance of the composition and increased viscosity thereof.
More precisely, the use of a flux (metal carbonate) makes it possible to limit the increase in viscosity while working the composition of the invention. The flux thus makes it possible to reduce the torque measured during the method by 14% to 22%.
More precisely, the metal carbonate serves initially as a flux (thereby reducing the viscosity of the composition), and subsequently, after being degraded in a fire, it serves to release both a metal oxide that reinforces the residue, and also to form a metal silicate from the inorganic filler, thereby improving the cohesion and the rigidity of the residue.
In addition, treating the metal carbonate with a metal salt makes it possible to reduce the hygrometry of the carbonate, thus improving shaping conditions.
Most advantageously, the compound further contains an inorganic filler that is porous or flaky.
The ternary metal hydroxide/inorganic filler/metal carbonate system treated by a metal salt serves to reinforce cohesion of the surface layer formed at the surface of the composition in the event of a fire by forming a residue that is vitreous on the outside but porous on the inside, such that the ability of the composition to withstand flame propagation is increased.
Advantageously, the inorganic filler may be selected from silica, talc, mica, graphite, and silica gel.
Also advantageously, the carbonate of a metal from group IA is potassium carbonate.
In an embodiment of the invention, the base polymer is an ethylene-vinyl acetate copolymer (EVA).
Other possible base polymers that may be used in the invention comprise in particular: polyethylene, polypropylene and copolymers thereof, silicones, elastomers, and thermoplastic elastomers. Suitable ethylene copolymers include: ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-propylene copolymers, and ethylene-alkyl acrylate copolymers. It is preferable to select a thermoplastic polymer and ethylene-vinyl acetate copolymer.
A preferred metal hydroxide for use in the invention is magnesium hydroxide Mg(OH)2. While it is being degraded, this composition releases about one-third of its weight as water, thereby locally cooling the medium, and then it forms a protective ceramized layer of metal oxide.
It is also possible to use alumina trihydrate Al(OH)3 as the metal hydroxide in the context of the present invention.
Other characteristics and advantages of the present invention appear in the following description of examples of compositions in accordance with the invention given by way of non-limiting illustration.
The invention applies equally well to binary and to ternary fireproofing systems.
In general, a ternary fireproofing system of the invention provides particularly high resistance to flame propagation since the mechanism which it implements leads to a complex assembly of layers that are particularly effective in withstanding fire being formed on the surface of the polymer-based material. This assembly of layers comprises in succession, going from the surface of the base material, and when the filler is silica, Mg(OH)2 metal hydroxide, and K2CO3 carbonate:
a layer of charring (i.e. the result of transformation into charcoal by incomplete combustion, also referred to as xe2x80x9ccarbonizationxe2x80x9d) based on Sixe2x80x94Oxe2x80x94C;
a layer of microporous silica;
a shell of vitreous silicate (Sixe2x80x94O-metal) surrounding the preceding layers; and
a ceramic shell of K2O and MgO outside the vitreous silicate shell.
When a composition of the invention is degraded under the effect of a flame, the following are formed:
an inorganic residue (microporous silica) that withstands fire and is suitable for controlling decomposition rate by trapping gases and protecting the base material from the effect of high temperature and from continued combustion;
charring that serves to provide good cohesion between the base material and the inorganic residue;
a ceramic covering on the surface of the inorganic residue serving to reinforce it; and
a vitreous lattice beneath the ceramic covering to improve its cohesion and to reinforce the barrier effect.
Examples I to III below give compositions in accordance with the invention. Example IV is a comparative example using a prior art composition based on magnesium hydroxide Mg(OH)2 alone as the fireproofing additive. Example I corresponds to a binary system in accordance with the invention, while Examples II and III corresponding to ternary systems in accordance with the invention.